In a turbojet, inlet air is compressed in a compressor before it is mixed with fuel and burned in a combustion chamber. Hot gases produced in the chamber then drive one or several downstream turbines and are then ejected. The turbojet also comprises a computer performing power regulation and general electronic management functions: for example, the computer manages the fuel flow, the condition of thrust bearings, discharge valves and systems for fixing turbojet guide vanes.
The compressor is generally separated into two parts: a low pressure (LP) compressor followed by a high pressure (HP) compressor. Moreover, each of these two compressors is generally composed of several stages, the last stage of the HP compressor is the stage directly followed by the combustion chamber. Since the role of the compressor is to compress air to optimise the speed, pressure and temperature at the inlet to the combustion chamber, it is vital to monitor the static pressure at the outlet from the final stage of the HP compressor. This measurement is used for control of the turbojet and fuel proportioning.
FIG. 1 diagrammatically represents a turbojet TB comprising a device for measuring the static pressure at the outlet S from the high pressure compressor CMP. To simplify the description, this pressure is referred to as PS3 in the remainder of the text. As shown in FIG. 1, the turbojet TB comprises particularly a computer CT and a duct CNL that transfers air from the outlet S of the HP compressor CMP to the computer CT. A pressure unit within the computer CT is used to measure and convert the air pressure routed through a pressure sensor CP. This information is then used for engine control and for troubleshooting.
Experience shows a non-negligible number of incidents due to an incorrect PS3 pressure measurement, for example slower acceleration than normal, loss of thrust or impossibility of reaching the requested thrust. These incidents usually occur when the aircraft demands high thrust, in other words during takeoff, during the climbing or approach phase and can lead to the crew deliberately stop the turbojet.
Therefore the pressure sensor CP is usually made redundant, to make the measurement more reliable. Two pressure sensors CP1, CP2 then measure the routed air pressure, and it is checked that the interval between the two measurements is not too large. If measurements are divergent, the two values are compared with a theoretical value of the pressure PS3 determined using a model implemented in the computer CT, so that the position of the pressure sensor can be identified.
However, although this test is suitable for detecting a malfunction of a sensor, it cannot help to detect a defect on a duct. A large number of defects can be observed on the duct, particularly:                A loose connection of the duct to the computer, frequently after washing of the turbojet during which the duct was removed        Presence of ice or water at the connection of the duct to the computer        Presence of ice or water within the duct        Perforations in the duct, for example due to recurrent friction with surrounding systems.        
All these defects will cause an underestimate of the pressure PS3. For example, a blocked or perforated duct will cause a head loss reducing the pressure experienced by the sensor downstream from the defect. The leakage flow depends on the static pressure at the outlet from the HP compressor, and the area of the leak. The escaping air flow increases with increasing pressure PS3 and with increasing size of the defect. The head loss also depends on the leakage flow. Therefore the head loss is greater when the defect is pronounced and/or the pressure PS3 is high.
At the present time, the only way that a defect in the duct can be detected is for a maintenance operator to make a visual inspection of the duct, either by chance during a maintenance operation or intentionally following an incident (deliberate or unintentional stop of the turbojet in flight, loss of thrust, impossible to start, etc.).